The present invention relates to a titanium article having a protective coating and a method of applying a protective coating to a titanium article, particularly to a titanium aluminide article having a protective coating and a method of applying a protective coating to a titanium aluminide article.
Titanium aluminide alloys have potential for use in gas turbine engines, particularly for turbine blades and turbine vanes in the low pressure turbine and compressor blades and vanes in the high pressure compressor and the combustion chamber diffuser section. The gamma titanium aluminides provide a weight reduction compared to the alloys currently used for these purposes.
However, titanium aluminide alloys and gamma titanium aluminide alloys will require environmental protective coatings, above a certain temperature, in a similar manner to conventional nickel base alloys or cobalt base alloys.
Conventional environmental protective coatings for nickel base alloys and cobalt base alloys include aluminide coatings, platinum coatings, chromium coatings, MCrAlY coatings, silicide coatings, platinum modified aluminide coatings, chromium modified aluminide coatings, platinum and chromium modified aluminide coatings, silicide modified aluminide coatings, platinum and silicide modified aluminide coatings and platinum, silicide and chromium modified aluminide coatings etc. Aluminide coatings are generally applied by the well known pack aluminising, out of pack, vapour, aluminising or slurry aluminising processes. Platinum coatings are generally applied by electroplating or sputtering. Chromium coatings are generally applied by pack chromising or vapour chromising. Silicide coatings are generally applied by slurry aluminising. MCrAlY coatings are generally applied by plasma spraying or electron beam physical vapour deposition.
Thermal barrier coatings include yttria stabilised zirconia and magnesia stabilised zirconia etc. Thermal barrier coatings are generally applied by plasma spraying or electron beam physical vapour deposition.
The MCrAlY coatings and aluminide coatings are intended to produce a continuous external alumina layer on the outer surface of the coatings. However, only an alpha alumina provides satisfactory oxidation resistance and alpha alumina is not readily formed below 1000xc2x0 C. Additionally there is a problem of interdiffusion between the MCrAlY coating and the titanium aluminide and the MCrAlY coating and aluminide coatings have poor fracture toughness due to the high levels of aluminium which make them brittle. Chromium coatings formed by chromising are intended to produce a continuous external chromia layer on the outer surface of the coating. However, chromising produces a diffusion zone in the titanium aluminide article which is porous and thus not protective.
Accordingly the present invention seeks to provide a novel protective coating for a titanium article and a novel method of applying a protective coating to a titanium article.
Accordingly the present invention provides a titanium alloy article having a protective coating on the titanium alloy article, the protective coating comprising a coating of austenitic steel.
Preferably the protective coating comprises a chromia layer on the austenitic steel coating.
Preferably the protective coating comprises a silica layer between the austenitic steel coating and the chromia layer.
Preferably the titanium alloy article comprises a titanium aluminide, more preferably the titanium alloy article comprises a gamma titanium aluminide, an alpha 2 titanium aluminide or an orthorhombic titanium aluminide.
Preferably a barrier layer is arranged on the titanium alloy article and the austenitic steel coating is on the barrier layer.
Preferably the barrier layer comprises silica, titanium nitride, titanium aluminium nitride or alumina.
Preferably the titanium alloy article comprises a turbine blade, a turbine vane, a compressor blade, or a compressor vane.
Preferably the austenitic steel comprises austenitic stainless steel.
The present invention also provides a method of applying a protective coating to a titanium alloy article comprising depositing a coating comprising austenitic steel onto the titanium alloy.
Preferably the method comprises forming a chromia layer on the austenitic steel coating.
Preferably the method comprises forming a silica layer between the austenitic steel coating and the chromia layer.
Preferably the method comprises depositing the austenitic steel coating by physical vapour deposition, chemical vapour deposition, low pressure plasma spraying, air plasma spraying, high velocity oxy fuel plasma spraying, cladding, hot isostatic pressing, or electroplating.
Preferably the method comprises depositing the austenitic steel coating by sputtering.
Alternatively austenitic steel coating may be deposited by direct laser fabrication. The titanium alloy article may be formed by direct laser fabrication.
The whole of the titanium alloy article may be formed by a direct laser fabrication and subsequently the austenitic steel coating is deposited on the titanium alloy article by direct laser fabrication.
Each layer of the titanium alloy article and the austenitic steel coating may be formed by sequentially forming a layer of the titanium alloy article by direct laser fabrication and depositing the austenitic steel coating on the layer of the titanium alloy article by direct laser fabrication.
Preferably the titanium alloy article comprises a titanium aluminide, more preferably the titanium alloy article comprises a gamma titanium aluminide, an alpha 2 titanium aluminide or an orthorhombic titanium aluminide.
Preferably the method comprises depositing a barrier layer on the titanium alloy article and depositing the austenitic steel coating on the barrier layer.
Preferably the barrier layer comprises silica, titanium nitride, titanium aluminium nitride or alumina.
Preferably the titanium alloy article comprises a turbine blade, a turbine vane, a compressor blade, or a compressor vane.
Preferably the austenitic steel comprises austenitic stainless steel.